There have already been several subtle hints (that could easily be missed or dismissed, but were there nevertheless) that Nick still has that obsessive nature that he previously had. The most recent (before Thursday's episode, anyway) was when he declared his love for Gabi, and she said that she felt the same way. To most people, that probably would have been enough, but Nick had to hear the actual words, "I love you."

It's also clear that Nick gets pissed off every time Will and Sonny are around -- Gabi thinks that it's just because of Nick's feelings about gay people, but (to me, anyway) it's obvious that Nick is also annoyed whenever Gabi's attention is not focused entirely on him.

Just speculation on my part at this point, but as far as I'm concerned, Nick hasn't changed -- he's still unbalanced, and it's only a matter of time before that part of his personality fully emerges again.